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Theory of Computing. Lecture 1 MAS 714 Hartmut Klauck. Organization:. Lectures: Tue 9:30-11:30 TR+20 Fr 9:30-10:30 Tutorial: Fr:10:30-11:30 Exceptions: This week no tutorial, next Tuesday no lecture. Organization. Final: 60%, Midterm: 20%, Homework: 20%

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theory of computing

Theory of Computing

Lecture 1

MAS 714

Hartmut Klauck

organization
Organization:
  • Lectures:Tue 9:30-11:30 TR+20Fr 9:30-10:30
  • Tutorial:Fr:10:30-11:30
  • Exceptions: This week no tutorial, next Tuesday no lecture
organization1
Organization
  • Final: 60%, Midterm: 20%, Homework: 20%
  • Homework 1 outon Aug. 15, due on Aug. 15
  • http://www.ntu.edu.sg/home/hklauck/MAS714.htm
books
Books:
  • CormenLeisersonRivest Stein: Introduction to Algorithms
  • Sipser: Introduction to the Theory of Computation
  • Arora Barak: Computational Complexity- A Modern Approach
computation
Computation
  • Computation: Mapping inputs to outputs in a prescribed way, by small, easy steps
  • Example: Division
    • Div(a,b)=c such that bc=a
      • How to find c?
      • School method
outline theory of computing
Outline: Theory of Computing

Algorithms

Cryptography

Complexity

Data Structures

Machine Models

Uncomputable

Formal Languages

Universality

Proof Systems

this course
This course
  • First half: algorithms and data-structures
  • Second half: Some complexity, computability, formal languages
    • NP-completeness
    • Computable and uncomputable problems
    • Time-hierarchy
    • Automata and Circuits
algorithms
Algorithms
  • An algorithm is a procedure for performing a computation
  • Algorithmsconsist of elementary steps/instructions
  • Elementary steps depend on the model of computation
    • Example: C++ commands
algorithms example
Algorithms: Example
  • Gaussian Elimination
    • Input: Matrix M
    • Output: Triangular Matrix that is row-equivalent to M
    • Elementary operations: row operations
      • swap, scale, add
algorithms1
Algorithms
  • Algorithmsarenamed after Al-Khwārizmī(AbūʿAbdallāhMuḥammadibnMūsā al-Khwārizmī)c. 780-850 cePersianmathematicianandastronomer
  • (Algebra is also named after hiswork)
  • His worksbroughtthepositionalsystemofnumberstotheattentionofEuropeans
algorithms example1
Algorithms: Example
  • Addition via theschoolmethod:
    • Write numbersundereachother
    • Add numberpositionbypositionmoving a „carry“ forward
  • Elementaryoperations:
    • Add twonumbersbetween 0 and 9(memorized)
    • Read and Write
  • Can deal witharbitrarilylongnumbers!
datastructure
Datastructure
  • The addition algorithm uses (implicitly) an array as datastructure
    • An array is a fixed length vector of cells that can each store a number/digit
    • Note thatwhenweadd x and y thenx+yisatmost 1 digitlongerthanmax{x,y}
    • So the result can be stored in an array of length n+1
multiplication
Multiplication
  • The schoolmultiplicationalgorithmis an exampleof a reduction
  • First we learn how to add n numbers with n digits each
  • To multiply x and y we generate n numbers xi¢ y¢2i and add them up
  • Reduction from Multiplication to Addition
complexity
Complexity
  • Weusuallyanalyzealgorithmsto grade theperformance
  • The mostimportant (but not theonly) parametersare time andspace
  • Time referstothenumberofelementarysteps
  • Space referstothestorageneededduringthecomputation
example addition
Example: Addition
  • Assumeweaddtwonumbersx,ywith n decimaldigits
  • Clearlythenumberofelementarysteps (addingdigitsetc) growslinearlywith n
  • Space is also ¼n
  • Typical: asymptoticanalysis
example multiplication
Example: Multiplication
  • Wegenerate n numberswithatmost 2n digits, addthem
  • Number of steps is O(n2)
  • Space is O(n2)
  • Much fasteralgorithmsexist
    • (but not easy to do withpenandpaper)
  • Question: Is multiplicationharderthanaddition?
    • Answer: wedon‘tknow...
our m odel of computation
Our Model of Computation
  • We could use Turing machines...
  • Will consider algorithms in a richer model, a RAM
  • RAM:
    • randomaccessmachine
  • Basicallywe will just usepseudocode/informal language
slide18
RAM
  • Random Access Machine
    • Storage is made of registers that can hold a number (an unlimited amount of registers is available)
    • The machine is controlled by a finite program
    • Instructions are from a finite set that includes
      • Fetching data from a register into a special register
      • Arithmetic operations on registers
      • Writing into a register
      • Indirect addressing
slide19
RAM
  • Random Access Machines and Turing Machines can simulate each other
  • There is a universal RAM
  • RAM’s are very similar to actual computers
    • machine language
computation costs
Computation Costs
  • The time cost of a RAM step involving a register is the logarithm of the number stored
    • logarithmic cost measure
    • adding numbers with n bits takes time n etc.
  • The time cost of a RAM program is the sum of the time costs of its steps
  • Space is the sum (over all registers ever used) of the logarithms of the maximum numbers stored in the register
other machine models
Other Machine models
  • Turing Machines (we will define them later)
  • Circuits
  • Many more!
  • A machine model is universal, if it can simulate any computation of a Turing machine
  • RAM’s are universal
    • Vice versa, Turing machines can simulate RAM’s
types of analysis
Typesof Analysis
  • Usually we will use asymptotic analysis
    • Reason: nextyear‘scomputer will befaster, so constantfactorsdon‘t matter (usually)
    • Understand theinherentcomplexityof a problem (canyoumultiply in linear time?)
    • Usuallygivestherightanswer in practice
  • Worstcase
    • The running time of an algorithmisthemaximum time over all inputs
    • On thesafesidefor all inputs…
types of analysis1
Types of Analysis
  • Averagecase:
    • Averageunderwhichdistribution?
    • Oftenthe uniform distribution, but maybeunrealistic
  • Amortizedanalysis
    • Sometimes after somecostlypreparationswecansolvemanyprobleminstancescheaply
    • Count theaveragecostof an instance (preparationcostsarespreadbetweeninstances)
    • Often used in datastructure analysis
asymptotic analysis
Asymptotic Analysis
  • Different models lead to slightly different running times
    • E.g. depending on the instruction set
  • Also computers become faster through faster processors
    • Same sequence of operations performed faster
  • Therefore we generally are not interested in constant factors in the running time
    • Unless they are obscene
slide25
O, , £
  • Let f,g be two monotone increasing functions that send N to R+
  • f=O(g) if 9n0,c8n>n0: f(n)· c g(n)
  • Example:f(n)=n, g(n)=1000n+100 )g(n)=O(f(n))
    • Set c=1001 and n0=100
  • Example:f(n)=n log n, g(n)=n2
slide26
O, , £
  • Let f,g be two monotone increasing functions that send N to R+
  • f = (g) iff g=O(f)
    • Definition by Knuth
  • f = £(g) iff [ f=O(g) and g=O(f) ]
  • o, !: asymptotically smaller/larger
  • E.g., n=o(n2)
  • But 2n2 + 100 n=£(n2)
some functions
Some functions

2n+10n2/500n log(n)/2